While the benefits of implementing virtually any high frequency electronic device using gallium arsenide (GaAs) are well known, doing so has traditionally required fabricating such GaAs based devices on a separate processing line from conventional silicon based devices.
One reason for this requirement is that ohmic contacts for GaAs based devices are traditionally fabricated using gold (Au), because non-Au metals do not form good ohmic contacts with n-type GaAs. Gold, however, contaminates silicon processing lines in a number of ways. Among the problems associated with gold contamination in silicon processing lines is the unwanted creation of several inter-metallic compounds (e.g., “white plaque” and “purple plaque”) that increase device resistance and often result in device failure. The use of gold based ohmic contacts is therefore an impediment to the ability to process (e.g., fabricate) GaAs based devices on silicon processing lines. Thus, non-gold ohmic contacts for GaAs based devices are needed.
Another reason GaAs based devices typically require a separate processing line is that microstrip transmission lines for GaAs based devices are typically fabricated with a ground-plane on the back-side of a GaAs wafer. Precision wafer thinning is then needed to control the transmission line impedance. Moreover, a back-side ground plan and through-substrate vias are typically needed. Traditional silicon processing lines, however, are not equipped for such processing techniques. Thus, transmission lines for GaAs based devices that require back-side processing and precision wafer thinning represent an impediment to the ability to process (e.g., fabricate) GaAs based devices on silicon processing lines. Thus, transmission lines fabricated with only front-side processing techniques are needed.